Full java runtime: Avoid allocating ArrayList iterators when serializing UnknownFieldSet

Use old-style for loop instead.

This should speed up processes that serialize many unknown fields, and reduce some GC pressure.

PiperOrigin-RevId: 638889708

java/core/src/main/java/com/google/protobuf/UnknownFieldSet.java

@@ -15,7 +15,6 @@ import java.util.ArrayList;
 import java.util.Arrays;
 import java.util.Collections;
 import java.util.List;
-import java.util.ListIterator;
 import java.util.Map;
 import java.util.TreeMap;
 
@@ -778,40 +777,52 @@ public final class UnknownFieldSet implements MessageLite {
     }
 
     /** Serializes the field, including field number, and writes it to {@code output}. */
+    @SuppressWarnings({"ForeachList", "ForeachListWithUserVar"}) // No iterator allocation.
     public void writeTo(int fieldNumber, CodedOutputStream output) throws IOException {
-      for (long value : varint) {
+      for (int i = 0; i < varint.size(); i++) {
+        long value = varint.get(i);
         output.writeUInt64(fieldNumber, value);
       }
-      for (int value : fixed32) {
+      for (int i = 0; i < fixed32.size(); i++) {
+        int value = fixed32.get(i);
         output.writeFixed32(fieldNumber, value);
       }
-      for (long value : fixed64) {
+      for (int i = 0; i < fixed64.size(); i++) {
+        long value = fixed64.get(i);
         output.writeFixed64(fieldNumber, value);
       }
-      for (ByteString value : lengthDelimited) {
+      for (int i = 0; i < lengthDelimited.size(); i++) {
+        ByteString value = lengthDelimited.get(i);
         output.writeBytes(fieldNumber, value);
       }
-      for (UnknownFieldSet value : group) {
+      for (int i = 0; i < group.size(); i++) {
+        UnknownFieldSet value = group.get(i);
         output.writeGroup(fieldNumber, value);
       }
     }
 
     /** Get the number of bytes required to encode this field, including field number. */
+    @SuppressWarnings({"ForeachList", "ForeachListWithUserVar"}) // No iterator allocation.
     public int getSerializedSize(int fieldNumber) {
       int result = 0;
-      for (long value : varint) {
+      for (int i = 0; i < varint.size(); i++) {
+        long value = varint.get(i);
         result += CodedOutputStream.computeUInt64Size(fieldNumber, value);
       }
-      for (int value : fixed32) {
+      for (int i = 0; i < fixed32.size(); i++) {
+        int value = fixed32.get(i);
         result += CodedOutputStream.computeFixed32Size(fieldNumber, value);
       }
-      for (long value : fixed64) {
+      for (int i = 0; i < fixed64.size(); i++) {
+        long value = fixed64.get(i);
         result += CodedOutputStream.computeFixed64Size(fieldNumber, value);
       }
-      for (ByteString value : lengthDelimited) {
+      for (int i = 0; i < lengthDelimited.size(); i++) {
+        ByteString value = lengthDelimited.get(i);
         result += CodedOutputStream.computeBytesSize(fieldNumber, value);
       }
-      for (UnknownFieldSet value : group) {
+      for (int i = 0; i < group.size(); i++) {
+        UnknownFieldSet value = group.get(i);
         result += CodedOutputStream.computeGroupSize(fieldNumber, value);
       }
       return result;
@@ -821,9 +832,11 @@ public final class UnknownFieldSet implements MessageLite {
      * Serializes the field, including field number, and writes it to {@code output}, using {@code
      * MessageSet} wire format.
      */
+    @SuppressWarnings({"ForeachList", "ForeachListWithUserVar"}) // No iterator allocation.
     public void writeAsMessageSetExtensionTo(int fieldNumber, CodedOutputStream output)
         throws IOException {
-      for (ByteString value : lengthDelimited) {
+      for (int i = 0; i < lengthDelimited.size(); i++) {
+        ByteString value = lengthDelimited.get(i);
         output.writeRawMessageSetExtension(fieldNumber, value);
       }
     }
@@ -854,17 +867,18 @@ public final class UnknownFieldSet implements MessageLite {
      * Serializes the field, including field number, and writes it to {@code writer}, using {@code
      * MessageSet} wire format.
      */
-    private void writeAsMessageSetExtensionTo(int fieldNumber, Writer writer)
-        throws IOException {
+    @SuppressWarnings({"ForeachList", "ForeachListWithUserVar"}) // No iterator allocation.
+    private void writeAsMessageSetExtensionTo(int fieldNumber, Writer writer) throws IOException {
       if (writer.fieldOrder() == Writer.FieldOrder.DESCENDING) {
         // Write in descending field order.
-        ListIterator<ByteString> iter = lengthDelimited.listIterator(lengthDelimited.size());
-        while (iter.hasPrevious()) {
-          writer.writeMessageSetItem(fieldNumber, iter.previous());
+        for (int i = lengthDelimited.size() - 1; i >= 0; i--) {
+          ByteString value = lengthDelimited.get(i);
+          writer.writeMessageSetItem(fieldNumber, value);
         }
       } else {
         // Write in ascending field order.
-        for (ByteString value : lengthDelimited) {
+        for (int i = 0; i < lengthDelimited.size(); i++) {
+          ByteString value = lengthDelimited.get(i);
           writer.writeMessageSetItem(fieldNumber, value);
         }
       }
@@ -874,9 +888,11 @@ public final class UnknownFieldSet implements MessageLite {
      * Get the number of bytes required to encode this field, including field number, using {@code
      * MessageSet} wire format.
      */
+    @SuppressWarnings({"ForeachList", "ForeachListWithUserVar"}) // No iterator allocation.
     public int getSerializedSizeAsMessageSetExtension(int fieldNumber) {
       int result = 0;
-      for (ByteString value : lengthDelimited) {
+      for (int i = 0; i < lengthDelimited.size(); i++) {
+        ByteString value = lengthDelimited.get(i);
         result += CodedOutputStream.computeRawMessageSetExtensionSize(fieldNumber, value);
       }
       return result;